Power supply switching circuits are widely used in modern electronic products such as flat panel displays. The power supply switching circuit is typically used for switching between two or more input voltage signals when the electronic product is in different working states. Generally, the power supply switching circuit is also capable of regulating the input voltage signals, so as to provide a desired output voltage signal for the electronic product.
FIG. 6 is a diagram of a conventional power supply switching circuit. The power supply switching circuit 100 includes a first input 110, a second input 120, a first diode 150, a second diode 160, a voltage regulator 140, and an output 130. The voltage regulator 140 is a direct current to direct current (DC-DC) regulator, which includes an input terminal 141 and an output terminal 142.
The first input 110 and the second input 120 are configured to receive a first voltage signal and a second voltage signal, respectively. The first diode 150 together with the second diode 160 are configured to switch the power supply switching circuit 100, so that the power supply switching circuit 100 receives a selected one of the first and second voltage signals. Positive terminals of the first and second diodes 150 and 160 are electrically coupled to the first input 110 and the second input 120, respectively. Both negative terminals of the first and second diodes 150 and 160 are electrically coupled to the input terminal 141 of the voltage regulator 140. The output terminal 142 of the voltage regulator 140 is electrically coupled to the output 130 of the power supply switching circuit 100. An electrolytic capacitor (not labeled) and a ceramic capacitor (not labeled) are electrically coupled in parallel between the voltage regulator 140 and ground.
In operation, the power supply switching circuit 100 has two working states. In a first working state, the first voltage signal is applied to the first input 110 and the second voltage signal is cut off. In this situation, the first diode 150 is in an on state and the second diode 160 is in an off state. The power supply switching circuit 100 is switched to receive the first voltage signal. Then the first voltage signal is regulated by the voltage regulator 140, and converted to a desired output voltage signal. Finally, the output voltage signal is outputted via the output 130.
In a second working state, the first voltage signal is cut off and the second voltage signal is applied to the second input 120. In this situation, the first diode 150 is in an off state and the second diode 160 is in an on state. The power supply switching circuit 100 is switched to receive the second voltage signal. Then the second voltage signal is regulated by the voltage regulator 140, and converted to a desired output voltage signal. Finally, the output voltage signal is outputted via the output 130.
A typical flat panel display, such as a liquid crystal display, employs the power supply switching circuit 100 to carry out the function of input signal switching and voltage regulation. In the power supply switching circuit 100, the first diode 110, the second diode 120, and the voltage regulator 140 are all essential elements. The DC-DC voltage regulator 140 is usually expensive. As a result, the cost of the power supply switching circuit 100 and the flat panel display employing the power supply switching circuit 100 are both high.
It is, therefore, desired to provide a power supply switching circuit and a flat panel display employing the power supply switching circuit that can overcome the above-described deficiencies.